It is well known to introduce chemical substances into a body in which the chemical substances contact body tissue. One obvious example is introducing medication in pill form into a human or animal. Aside from the particular active ingredient, the pill may comprise different types of waxes, fats, and filler or wetting agents, all of which must not react with body tissue. One such example can be found in U.S. Pat. No. 4,994,227 to Dietz et al.
In medical procedures such as surgery, it is known to introduce chemical substances into a body cavity and in contact with vital tissues. One such example is disclosed in U.S. Pat. No. 5,093,319 to Higham et al. where adhesion or inflammation of tissues after surgery is prevented by placing a material made up of biodegradable derivatives of chitin between the tissues. Another example of the prevention of post-operative adhesions is found in U.S. Pat. No. 5,266,326 to Barry et al. where polysaccharides, such as alginates, and a complexing agent, such as calcium chloride, are combined in situ between affected tissues to form a biodegradable gel. U.S. Pat. No. 5,405,366 to Fox et al. teaches the production of cohesive, non stringy cross-linked gels for the delivery of therapeutic drugs to wound sites by subjecting solutions, such as that of polyethylene oxide in combination with other compounds, to high energy radiation.
With some current surgical procedures, as the uses and technology of medical ultrasound imaging have evolved, imaging procedures that were once performed externally over skin surfaces are now being performed in contact with organs, tissue and body cavity mucosa. For example, when imaging the liver during surgery, the transducer is often placed directly on its surface.
Ultrasound, as used for medical applications, utilizes high frequencies, typically between 1 and 20 MHz for imaging and flow measurements, which are poorly transmitted by air and requires a coupling or conduction medium similar in acoustic properties to tissue, commonly a thick fluid or gel, to transfer the acoustic energy between the body and the electronics. The ultrasound coupling gel or fluid displaces air and fills contours between the piezoelectric transducer or “eye” of the instrument, which converts energy between electrical and acoustic, and the body into which the sound is being directed. This gel or fluid material, by nature of its physical and acoustic properties, serves as an ultrasound acoustic coupler between the body and the electronic transducer, thereby acoustically joining the two, so that the ultrasound based information developed, can freely pass back and forth between the body and the transducer. The gel or fluid material may also serve as a lubricant to aid in the introduction of a medical device used for imaging, such as endoscopes, into the body.
Because of the coupling effect, this media is commonly referred to as an ultrasound couplant, ultrasound gel, ultrasound transmission media or acoustic transmission media. Many fluids and water-based gels have been used as ultrasound couplants over the years. Early use of mineral oil was replaced by gels of water and acrylic based polymers such as CARBOPOL® (a registered trademark of BF Goodrich Specialty Chemicals) typical of those described in U.S. Pat. No. 4,002,221 to Buchalter, and also gels made from acrylic polymers and attached as coupling members to transducers such as are described in U.S. Pat. No. 4,459,854 to Richardson et al. as a method for improvement of perivascular blood flow measurement.
Use of currently available ultrasound coupling fluids and gels of prior art in surgical, and ultrasound guided needle puncture procedures have fundamental disadvantages that place the patient at risk. Some of these disadvantages are described below:    1. Oils or thickened water-based gels typically used in medical ultrasound are similarly described as in previously discussed U.S. Pat. No. 4,002,221, and are comprised of chemical compounds such as acrylic polymers, carboxy alkyl cellulose, hydroxyethylcellulose, carboxy polymethylene, polyalkylene glycol humectants, organic acids, alkali metal salts, parabens and other germicidal and fungicidal agents, and surfactants that are unsuitable for use in applications where they may be carried into the body tissue or fluids.    2. The above-mentioned couplants are also commercially available in sterilized form, thus implying and encouraging their inappropriate use in vivo where their chemical constituents are either known to be harmful to the human body or have not been evaluated for their in vivo use.    3. Currently available ultrasound couplants supplied in sterile form contain acrylic polymers such as CARBOPOL as a common and primary ingredient. CARBOPOL, for example, has not been tested for in vivo biocompatibility. Some currently available sterile couplants also contain cellulose ethers to increase salt stability. According to E. Doecker in “Water Swollen Cellulose Derivatives in Pharmacy” from Hydrogels in Medicine and Pharmacy: Vol. 2-Polymers, edited by Peppas N. A., CRC Press Inc., Boca Raton, Fla., 1987, pg. 124, “in common use, such celluloses are used orally and externally, however parenteral administration of celluloses is not recommended since derivatives are not easily metabolized”. Since various chemicals of these formulations are not in vivo biocompatible, they can remain in the body as substances that can cause inflammation, disruption in flow of lymph, irritation, anaphylactic shock and other immune system reactions. This concern becomes apparent during ultrasound guided needle biopsy or aspiration, or inside the body when ultrasound transducers encapsulated in fragile sheaths containing sterile ultrasound couplant are inserted for imaging during surgery in direct contact with organs, tissue and blood.
Of additional concern are the unknown chemical constituents formed during sterilization processing. Methods of couplant sterilization include steam autoclave, E-beam, broad spectrum light and gamma radiation protocols. Couplant products that incorporate CARBOPOL in the formulation can break down as a result of heat during autoclaving. When exposed to ionizing radiation, such as in the case of gamma and E-beam, and high intensity light sterilization, free radicals can be formed, and chain scission and cross linking of the polymer can occur, as evidenced by presence of bubbles and changes in color, viscosity and mechanical properties of the polymer products.
It is important to note that sterility of a substance does not guarantee that it is biocompatible, or of greater importance, in vivo biocompatible. When a substance is sterile, it does not contain live microorganisms; however, such sterile materials may not be in vivo-biocompatible should they contain compounds that are incompatible with tissue or body fluids. For example, natural and synthetic materials that are recognized by the FDA as GRAS (Generally Regarded As Safe), may not be in vivo biocompatible. An in vivo biocompatible substance is both sterile, containing no living micro-organisms, and contains no chemicals or substances that are toxic or cause inflammation or immune system reactions, such as from pyrogens, within the living human body. A substance such as the device of this invention is in vivo biocompatible as an ultrasound couplant in contact with human tissue and body fluids.    4. In instances where sterile latex rubber or synthetic “sheaths” containing thickened chemical ultrasound coupling gels are used to encapsulate the ultrasound transducer during surgery, such as described in U.S. Pat. No. 5,259,383 to Holstein et al.; U.S. Pat. No. 4,593,699 to Poncy et al. and U.S. Pat. No. 5,676,159 to Navis; tearing, cutting, or rupture of the sheath can result in the bio-incompatible ultrasound couplant spilling into the body cavity. During procedures such as transcutaneous biopsy or aspiration of fluid under ultrasound imaging guidance, such bio-incompatible ultrasound couplants of the prior art are placed directly on the skin covering the area of the biopsy. A biopsy needle can carry such chemicals into the body, such as into the breast or into amniotic fluid.
It is an object of the present invention to provide an ultrasound couplant and device lubricant suitable for the medical use of ultrasound acoustic energy for imaging and doppler based flow measurement, while contacting body tissue, fluids and organs, during transcutaneous biopsy and fluid aspiration, and to lubricate the passage of the imaging device into body cavities.
It is a further object of the present invention to provide gels and fluids that are in vivo biocompatible, and suitable for use in diagnostic ultrasound procedures inside the body of a human during surgery, guided biopsy, within body cavities and ophthalmic imaging.